2008 August notes: Observing statistics, 2004 SB56, 2007 RT12, 2001 QL153, 2008 PG1

The weather was very poor at Great Shefford in July and August. With 25 hours at the telescope, July provided less than half the observing hours logged for that month in any of the last three years. August was officially the cloudiest August in the UK since records began in 1929 and provided only about 50 (generally poor quality) hours, again well below totals from recent years.

However, the night of August 3/4th provided the opportunity to recover three NEOs not seen since their discovery apparitions and a somewhat unusual confirmation of a NEO Confirmation Page (NEOCP) object.

Aten 2004 SB56 had sped northwards after conjunction with the Sun in July and in the 10 days before recovery had brightened 3 magnitudes and increased elongation from 35 to 72°. With an uncertainty area on the sky of 14' it was picked up only 3' from prediction at mag +16.7 and about 6° from the North Pole, a relatively easy recovery.

Apollo 2007 RT12 had an uncertainty of about 1° and at mag +20.0 was a potentially difficult object to recover, just 1° east of Alpha Cep, deep in the Milky Way. Fortunately it was only 8' from prediction and not involved with any field stars during the time it was imaged. With an absolute magnitude of +23.9, equating to an estimated diameter of 30-95 meters, there have only been 10 other NEOs recovered at a second opposition that are smaller than 2007 RT12. Included in this set are the Earth co-orbital companion 2002 AA29 and the tiny 2006 RH120 that was temporarily captured in Earth orbit during 2006 & 2007. My thanks to Sergio Foglia for providing a list of small NEOs seen at more than one apparition.

The third object was 2001 QL153, an Amor that had been observed for about 4 months and had not been seen since January 2002. It too had an uncertainty of about 1° and predicted to be mag +20.5 and could have posed a problem to pick up. It was in a less crowded field than 2007 RT12 and after more than six years since last being observed was recovered only 9' from prediction.

A LINEAR discovery put on the NEOCP on August 3rd turned out to be something of a puzzle. 2008 PG1 was observed from 9-10am UT on August 3 by LINEAR at mag +19, moving at 9"/min from west to east, in the morning sky about 13° north of Mira Ceti. By 2am on August 4th it was predicted to be accelerating and to have an uncertainty area 1.7° long, extending roughly West to East, in the direction of motion. I eventually confirmed it 43' further west than the most westerly point of the uncertainty area (quite unusual for the MPC prediction to be this far out) and moving at about the same speed as when LINEAR discovered it. The orbit appears to be rather indeterminate but it was announced on MPEC 2008-P14 with just the observations from LINEAR and my set of confirmation positions. The orbital eccentricity had been assumed (done sometimes when the orbit is rather indeterminate, to try and help improve the orbit solution). No other observatories had reported positions for the NEO before I got my next chance to observe the area early on August 9th. By then, using FindOrb software from Bill Gray to generate a large number of 'Monte Carlo' orbits to estimate the likely uncertainty area, it looks like it could have been anywhere in a nearly 3° long band. I didn't manage to locate it that morning nor on my next opportunity 4 days later and it has not been reported from anywhere else since. It is now listed as a Virtual Impactor by JPL with 97 potential impacts, the first in only 5 years time, but with such large uncertainties in its orbital elements the chance of collision then is given as just 1 in 200 million!

(Update 10 May 2012: 2008 PG1 was linked to 2009 EV by the Minor Planet Center, read more here)

2008 June notes: 2008 LA, 2008 LG2, 2008 LH2, 2008 LB, 2003 OB4

After a rather lack-lustre May, June provided the best stats I've logged for that month since my records started in 2002, with 16 at least partially usable nights and 47 hours at the telescope.

This time of year, activity on the NEO Confirmation Page is tailing off as the main NASA survey sites in the southern USA get closer to the summer monsoon season, even so, there were 22 new NEO discoveries announced and 9 of those were observed, with 2008 LA and 2008 LG2 being followed on 5 nights each and still being listed (July 5th) as 'virtual impactors'. Another NEO 2008 LH2 was confirmed soon after LINEAR discovered it, just after its closest approach to 9 Lunar Distances (LD) and followed for a total 7 nights.

2008 LB was the closest and fastest moving NEO seen during the month, a difficult Apollo object, last recorded 16 hours before its closest approach at about 3.8 LD, mag +17.9 and moving at 83"/min through the very crowded star fields just 5° south-east of the Scutum star cloud.

Another object recorded in a rich star field was 2003 OB4, this Amor type NEO had been discovered by NEAT on 23 July 2003 and, with prediscovery images, had been tracked for three months that year but not seen since. I searched for it on several nights in eastern Ophiuchus and found it over 0.5° off-track on 9th June at mag +19, confirming it the next night, with the recovery being announced in MPEC 2008-L43.

2008 May notes: 2008 JL24

After a promising opening week, May turned out to be yet another disappointing month, with just a handful of (cloud interrupted) nights.

The most interesting object observed during the month was 2008 JL24, a mag +17.5 LINEAR discovery added to the NEO Confirmation Page (NEOCP) just before midnight on 11th May and predicted to be moving at about 25"/min. The sky cleared at Great Shefford a couple of hours later and I started searching for it at 00:25 UT. The uncertainty area indicated by the Minor Planet Center was about 80' wide, much bigger than my 18'x18' field of view. I started imaging at the nominal position, at the western end of the area and while the first set of images was being taken I checked with FindOrb whether by chance any of the astrometric positions submitted by LINEAR had poor residuals, in case I could possibly improve the prediction. All the positions looked good, but the ephemeris generated by FindOrb had the object way off to the east, beyond the furthest eastern extent of the NEOCP uncertainty area.

I decided to start searching from the FindOrb prediction and to work my way back to the western side of the NEOCP area with overlapping fields. I picked up the NEO in the corner of the second field attempted, over 1.5 degrees from the nominal position, but in fact only 10 minutes after I had started observing! I re-centred and obtained some further images and sent off the astrometry to the Minor Planet Center.

Initial uncertainty area for 2008 JL24.
White cross is MPC expected position, orange and red dots indicate MPC expected uncertainty area, yellow boxes are telescope fields originally planned to cover MPC uncertainty area.
4 coloured boxes on left show the search fields taken, with the actual position of 2008 JL24 marked with a red cross. Arrow length indicates the motion of the NEO in 1 hour.
Sky chart and field boxes plotted using SkyMap Pro 9.

When LINEAR discovered 2008 JL24 it was just inside the orbit of the Moon, at 0.94 Lunar Distances (LD) and by the time I confirmed it 18 hours later it had already receded to 1.4 LD. Over the years LINEAR has discovered a number of fast moving objects that were probably closer than the Moon at the time, but never picked up again and in fact 2008 JL24 is the first time that a NEO has been discovered while closer than the Moon and then subsequently confirmed. It was well observed over the next five days before fading out of sight just before the full Moon.



 

2008 April notes: 2008 GM2, 2008 GF1

After a good run of more than a week of clear nights in early April, the rest of the month brought rather poor weather, only a couple of nights in the month were were completely cloud free.

Two NEOs (both discovered during April with telescopes of the Catalina Sky Survey) came closer to Earth than the Moon, 2008 GM2 was discovered three days after its close pass, so had already receded to about 2.7 Lunar Distances (L.D.) before being picked up. I recorded it about 14 hours after discovery at about 3.3 L.D. but already it had slowed to a very sedate apparent speed of 6"/min.

The other was 2008 GF1, discovered on April 5th just over two days before closest approach and last caught from Great Shefford just before midnight, late on April 6th when it was moving at 70"/min and at mag +17.6, just over 2 L.D. away, about 12 hours before passing Earth at 0.8 L.D. Unfortunately, it was not picked up again, favourable locations for observing the fly-by being in central Asia, so both these NEOs went unobserved when at their closest.

2008 March notes: 2008 EZ7

Observing was limited in March, a combination of poor weather with very few nights keeping clear throughout and also the telescope being out of operation for a week, the problems eventually being tracked down to a failing mains transformer (see investigation in progress).

On 7th March (four days before the telescope broke down!) the Apollo 2008 EZ7 was discovered from the Siding Spring Observatory in Australia. From its accelerating motion it was obviously already very close to Earth and it was put on the NEO Confirmation page later that day. With a rapidly expanding uncertainty area where the new fast moving object was predicted to be I was fortunate enough to be able to confirm it later that evening at magnitude +17, at an altitude of less than 13 degrees, shortly before it disappeared into trees.

The next night it was to pass only 0.419 Lunar Distances (100,000 miles) from Earth, closest approach being at 01:21UT on 9th March. The first half of the night was cloudly but with breaks expected later in the morning. I started to try for it through gaps in the clouds at 01:47UT at which time it was moving at over 630"/minute. On the first low resolution shot taken to position the telescope, there it was, very obvious as a strong 30" long streak in the 3 second exposure. However, for the next half an hour the clouds stopped me getting anything other than more positioning shots, but eventually it was recorded in a 30 second gap in the clouds, enough to get eight 1 second exposures off. At that point it was magnitude +13, moving at 623"/minute and only about 102,000 miles from Earth.

The eight (rather noisy) images have been combined here with 2007 EZ7 moving from right to left. The field of view is 18'x18' and there is a 2 second gap between each of the exposures.

A few minutes later, when repositioning the telescope for the next attempt I recorded 2007 EZ7 streaking along as before, but also with another very similar streak in the field of view. This turned out to be artifical satellite 1985-048C=15825U, moving at about 900"/minute and more than four times closer than the NEO. I wonder how many NEOs on very close approaches are assumed to be artificial satellites in CCD images?

2007 EZ7 is to the right and 1985-048C is the slightly longer streak to the left below.

At the time of writing there have only been two other NEOs (with well determined orbits) that have been observed closer to Earth than 2007 EZ7, those being 2004 FH and 2006 DD1, but no doubt, with the pace of NEO discoveries at the moment, it is only a matter of time before another is seen to fly past even closer. Just three weeks later 2008 FP came closer than the Moon and was observed by the Catalina Sky Survey only slightly further out than 2007 EZ7, at a distance of 106,000 miles.